Lipid kinases catalyse the phosphorylation of lipids to produce species involved in the regulation of a wide range of physiological processes, including cellular migration and adhesion. The PI3-kinases are membrane associated proteins and belong to this class of enzymes which catalyse the phosphorylation of lipids which are themselves associated with cell membranes. The PI3-kinase delta (δ) isozyme (PI3 kinase δ) is one of four isoforms of type I PI3 kinases responsible for generating various 3′-phosphorylated phosphoinositides, that mediate cellular signalling and have been implicated in a number of cellular processes such as inflammation, growth factor signalling, malignant transformation and immunity [See Review by Rameh, L. E. and Cantley, L. C. J. Biol. Chem., 1999, 274:8347-8350].
Involvement of PI3 kinases in controlling inflammation has been confirmed in several models using pan-PI3 kinase inhibitors, such as LY-294002 and wortmannin [Ito, K. et al., J Pharmacol. Exp. Ther., 2007, 321:1-8]. Recent studies have been conducted using either selective PI3 kinase inhibitors or in knock-out mice lacking a specific enzyme isoform. These studies have demonstrated the role of pathways controlled by PI3 kinase enzymes in inflammation. The PI3 kinase δ selective inhibitor IC-87114 was found to inhibit airways hyper-responsiveness, IgE release, pro-inflammatory cytokine expression, inflammatory cell accumulation into the lung and vascular permeability in ovalbumin-sensitized, ovalbumin-challenged mice [Lee, K. S. et al., J. Allergy Clin. Immunol., 2006, 118:403-409 and Lee, K. S. et al., FASEB J., 2006, 20:455-65]. In addition, IC-87114 lowered neutrophil accumulation in the lungs of mice and neutrophil function, stimulated by TNFα [Sadhu, C. et al., Biochem. Biophys. Res. Commun., 2003, 308:764-9]. The PI3 kinase δ isoform is activated by insulin and other growth factors, as well as by G-protein coupled protein signaling and inflammatory cytokines. Recently the PI3 kinase dual δ/γ inhibitor TG100-115 was reported to inhibit pulmonary eosinophilia and interleukin-13 as well as mucin accumulation and airways hyperesponsiveness in a murine model, when administered by aerosolisation. The same authors also reported that the compound was able to inhibit pulmonary neutrophilia elicited by either LPS or cigarette smoke [Doukas, J. et al., J Pharmacol. Exp. Ther., 2009, 328:758-765]
Since it is also activated by oxidative stress, the PI3 kinase δ isoform is likely to be relevant as a target for therapeutic intervention in those diseases where a high level of oxidative stress is implicated. Downstream mediators of the PI3 kinase signal transduction pathway include Akt (a serine/threonine protein kinase) and the mammalian target of rapamycin, the enzyme mTOR. Recent work has suggested that activation of PI3 kinase δ, leading to phosphorylation of Akt, is able to induce a state of corticosteroid resistance in otherwise corticosteroid-sensitive cells [To, Y. et al., Am. J. Respir. Crit. Care Med., 2010, 182:897-904]. These observations have led to the hypothesis that this signalling cascade could be one mechanism responsible for the corticosteroid-insensitivity of inflammation observed in the lungs of patients suffering from COPD, as well as those asthmatics who smoke, thereby subjecting their lungs to increased oxidative stress. Indeed, theophylline, a compound used in the treatment of both COPD and asthma, has been suggested to reverse steroid insensitivity through mechanisms involving interaction with pathways controlled by PI3 kinase δ [To, Y. et al., Am. J. Respir. Crit. Care Med., 2010, 182:897-904].
At present the mainstay of treatment for both asthma and COPD is inhaled therapy, using a combination of corticosteroids, muscarinic antagonists and β2-agonists, as judged clinically appropriate. One way of addressing the unmet medical needs in COPD and asthma is to identify new therapeutic agents, for example suitable for use as inhaled medicines, which have the potential to provide significant benefit when used as a monotherapy or in combination with one or more medicaments from these three pharmacological classes. Therefore, there remains a need to identify and develop isoform selective PI3 kinase inhibitors which have the potential to provide enhanced therapeutic efficacy in asthma, COPD and other inflammatory diseases.